This disclosure addresses digital detection and analysis of RF pulses, for example radar pulses when the pulse parameters (duration, frequency and span) are varying and unknown. It is assumed that pulses of variable duration, frequency and span do not overlap and are confined within a wide sub-band. In many applications, incoming pulses have to be detected and pulse parameters (amplitude, phase and frequency) need to be determined in real or near-real time. Typically, RF pulses in a bandwidth of interest may come at random time intervals or have a low duty cycle, and, as a result, RF pulse triggering is required in order to process and/or store only relevant pulse data.
A conventional method for digital detection and measurement of RF modulated pulses is based on down-conversion. Analog down-conversion systems based on frequency mixers and low-pass filtering are well known. However analog down-conversion implementations have many disadvantages compared to digital implementations. For example, detection bandwidth and down-conversion frequency cannot be easily changed due to hardware limitations and local oscillator settling times. Analog frequency mixers are known to have stability issues, phase noise and non-linearity, resulting in distortions of down-converted signals.
Digital down-conversion (DDC) methods are based on analog to digital conversion of an incoming signal followed by numerical oscillator mixing, low pass filtering and down sampling. Such methods are known to have high stability, accuracy and flexibility compared to analog implementations. Principles of digital down-conversion are described in J. Alter and J. Coleman “Radar Digital Signal Processing” published in “Radar Handbook” by M. Skolnik, McGraw Hill, 2008. Similar methods are described in “Fundamentals of radar signal processing” by M. A. Richards (Mc.Graw-Hill, 2014, pp. 133-137).
In order to use digital down-conversion for RF signal detection and measurement of pulse parameters, parameters of the pulse (frequency span, center frequency) need to be known in advance. If the center frequency and span are variable, more complicated detection schemes are required. This problem has been addressed in a number of patents and publications.
U.S. Pat. No. 7,652,619 to Hibbard et al., entitled “Systems and Method Using Multiple Down-Conversion Ratios in Acquisition Windows,” discloses a system for performing different sampling rates on incoming signals in order to perform down-conversion with different timing resolution, obtaining high and low resolution pulses. High resolution signals are used for far-range applications, while low resolution signals are used for low-range data processing.
U.S. Pat. No. 7,941,111 to Cutler et al., entitled “Method and System for Detecting an RF Signal,” discloses a system for digitizing and storing data from multiple RF receivers. A separate trigger circuit determines whether an RF signal has been detected, and selected digitized data is transmitted to a central processing device for demodulation.
U.S. Pat. No. 8,644,429 to Krishnan et al., entitled “Digital Down Conversion and Fast Channel Selection of Narrowband Signals Using a Wide Band RF Tuner,” discloses a receiver which demodulates an input signal in a wide band. At a second stage, a specific narrow band signal is selected by a spectrum selection control unit and this narrow-band signal is demodulated.
U.S. Pat. No. 8,803,730 to Jiang, entitled “Radar Pulse Detection Using a Digital Radar Receiver,” discloses a system and apparatus for radar pulse detection operating in a dense electronic environment. In order to separate multiple radar signals, a procedure called “digital channelization” is used. The “digital channelization” is based on applying a bank of band-pass filters in order to detect signals in multiple frequency bands. However, implementing the bank of band pass digital filters is challenging for high speed signal processing. The method proposed in this patent is based on using high speed analog to digital conversion, down-sampling this signal and applying a digital band pass filter bank for the down-sampled signals. Then, the outputs of the narrow-band signals are combined in a decision-logic algorithm which determines the presence of radar pulse in the selected band or several bands.
U.S. Patent Application Publication No. 2013/0094616 to Laporte, entitled “Digital Down Conversion and Demodulation,” discloses a method and apparatus for down-conversion and demodulation of RF signal. A sampling frequency is chosen based on a bandwidth of interest. This application is applicable to wireless communication with multiple detection bands, and does not address multi-stage demodulation.
European patent EP 2,533,057 to Martin, entitled “Interleaved Digital Down-Conversion on a Test and Measurement Instrument,” discloses down-conversion performed on multiple channels of an input signal, so each channel is down-sampled and processed at low rate. This allows parallel computation of band-pass signals given a plurality of memory and down-conversion operations.
European patent EP 1,464,111 to Fernandez-Corbaton et al., entitled “Multiple Analog and Digital Down-Conversion,” discloses a method for down-conversion. The patent discloses a two-stage down conversion method, combining a first analog stage with a wide bandwidth and a second digital stage having a narrower bandwidth. The frequencies of the first and second down-conversion oscillators can be adjusted after an initial down-conversion, based on error signals.
However, none of the prior art patents and application do address the issue of optimal detection and estimation of RF pulse parameters. U.S. Pat. No. 8,644,429 and US Patent Application Publication No. 2013/0094616 disclose adjustment of down-conversion frequency, but require one or several narrow band filters, based on the assumption that the detected signal is continuous; those patents do not address pulse parameter estimation. Similarly, the methods described in U.S. Pat. No. 7,652,619 and U.S. Pat. No. 7,941,111 describe either variable sampling rate or multiple RF receiver implementations, and are not relevant to a standard digital down-conversion implemented with a fixed sampling rate ADC. U.S. Patent Application Publication No. 2014/0213197 allows optimal selection of a demodulation bandwidth; however, that application is related to a continuous wave communication system, assuming that the signal of interest is present in the bandwidth continuously, and does not discuss either down-conversion or pulse parameter estimation.
U.S. Pat. No. 8,803,730 and EP Patent No. 2,533,057 require a multi-bank set of filters, and address detection of signals in multiple narrow bands. These patents allow detection of multiple radar pulses in terms of envelope and duration; however, estimation of phase and frequency is not addressed.
Finally, EP Patent No. 1,464,111 is directed to continuous communication applications when a receiver has to be accurately tuned to the transmitter in a dense communication environment. Such methods are not applicable to real time detection and do not address pulse parameter estimation.
Therefore, the above-cited prior art methods do not address the most important issues for detection of RF pulses with unknown parameters: real time RF pulse triggering, and double-stage digital down conversion with optimal down-conversion parameters.